I. Field of the Invention
The present invention relates generally to the field of visual display devices, and more particularly to a pixel circuit of a display.
II. Background of the Invention
A visual display device constitutes one part of the functional modules in almost every electronic apparatus and plays an important role in facilitating human-machine interactions with that apparatus. It helps users to read information from the apparatus via the display device and further to control the apparatus operation. As newer generations of display devices continue to be developed, they are becoming both thinner and lighter. Display technology has progressed from conventional Cathode Ray Tube (CRT) displays to flat-panel display devices such as liquid crystal displays (LCD) or organic light emitting displays (OLED), which take advantage of advances in photoelectron and semiconductor manufacturing technologies.
In particular, active matrix organic light emitting diode (AMOLED) display technology has attracted a lot of attention and is subjected to intense research. AMOLED displays utilize transistors, for example implemented by thin-film transistor (TFT) techniques, to drive the organic light emitting diode. AMOLED displays conventionally include a mesh of scan and data lines that defines an array of pixels, each of which has one light-emitting device. The light-emitting device is usually driven by a pixel circuit associated to each pixel. In order to control individual pixels, a specific pixel is commonly selected via a scan line and a data line, and an appropriate operating voltage is also provided, so as to display the display information corresponding to each pixel.
FIG. 1 is a schematic diagram that illustrates a conventional 2T1C (two transistors and one capacitor per pixel) pixel circuit of an AMOLED.
As shown in FIG. 1, the pixel circuit includes a data transistor 11, a driving transistor 12, a storage capacitor 13 and a lighting device 14. The transistors can be any type of transistor, such as a thin film transistor or the like. For example, the data transistor 11 can be a n-type metal-oxide-semiconductor (NMOS) transistor and the driving transistor 12 can be a p-type metal-oxide-semiconductor (PMOS) transistor in the following descriptions. The data transistor 11 has a gate electrode connected to a scan line and a first source/drain electrode connected to a data line. The driving transistor 12 has a gate electrode connected to a second source/drain electrode of the data transistor 11 and a first source/drain electrode connected to a power source VDD. The storage capacitor 13 is connected to between the gate electrode of the driving transistor 12 and the first source/drain electrode of the driving transistor 12. The lighting device 14 has an anode electrode connected to a second source/drain electrode of the driving transistor 12 and a cathode electrode connected to a ground level.
During operation, a high voltage level scan signal turns on the data transistor 11, which enables the data signal to charge the storage capacitor 13. The voltage potential that stores within the storage capacitor 13 determines the magnitude of the current flowing through the driving transistor 12, so that the lighting device can emit the light based on the current. As to the conventional driving method mentioned above, the driving transistor 12 and the lighting device 14 are all kept in an activation state both at programming and display stages. Therefore, deviation of the driving voltage of the lighting device 14 is generated which impacts the display quality.
However, it is difficult to consistently maintain the luminance of a display due to the following disadvantages of the conventional pixel circuit. (1) The stored voltage potential of the storage capacitor 13 during the programming stage may not be accurate due to the IR drop of the power line, which extends from the power source VDD to the driving transistor 12. In the programming stage, the voltage potential of the storage capacitor 13 is determined by the voltage difference between the data line and the first source/drain electrode of the driving transistor 12, which connected to the voltage source VDD. Since the voltage at the first source/drain electrode of the driving transistor 12 may vary from that of other pixel circuits, the voltage potential stored in the storage capacitor 13 may not be accurate. (2) The clock feed-through effect may occur while the data transistor 11 is being turned off, such that the voltage potential of the storage capacitor 12 is altered.
Therefore, there is a need for an alternative 2T1C pixel circuit design that could solve or improve the above-mentioned drawbacks.